Bob...if I understand your question, do you want the choice to be shown to the user as an option to click 'full screen' when hover the mouse over an image?

Option Alex gave is one way to do it. This will show the user all the options (s / m/ l / xl/xxl/original)...but still the user has to click it. But if your idea is to display the picture full screen right when they enter your site, then you are up for some coding (should not be complex).